This invention relates to microwave (including millimeter wave) devices, and, more particularly, to microwave phase modulation with liquid crystals.
The index of refraction of many media is fixed and essentially constant, but there are "birefringent" media where the index of refraction of the material varies with relative orientation of a polarized wave and the medium. The birefringence of a uniaxial crystal is manifested by double angular displacements of a wave that passes through the medium. For example, in many viewing angles calcite crystals are birefringent to visible light, so that two differently positioned images are often seen when an object is viewed through a calcite crystal.
The index of refraction of some birefringent materials and of Kerr-effect isotropic materials can be controllably varied directionally by the application of a magnetic or electrical field to the medium. For example, liquid crystals are known to be birefringent for visible light, and variable under the application of an electric field. This property has been used in liquid crystal light valves to make color projection displays.
A liquid crystal is a state of matter intermediate between that of a liquid and a solid crystal, and having some properties of each. The liquid crystals are a liquid in the sense that they are a condensed phase that is flowable. They exhibit quasi-crystalline behavior in that their molecules may exist in an oriented arrangement wherein many molecules are aligned with respect to each other.
Liquid crystals have been known for more than 100 years, with particularly extensive development of new materials and devices in the last 30 years. The following references provide a general background to the structures, classifications, nomenclature, and applications of liquid crystals: "Liquid Crystals", Proc. of Inter. Liquid Crystal Conf., Bangalore, India, Ed. by S. Chandrasekhar (Bangalore Book Printers, 1973); G. H. Brown et al., "A Review of the Structure and Physical Properties of Liquid Crystals, Ed. by B. J. Starkoff (The Chemical Rubber Company by CRC Press, 1971); "Liquid Crystals, The Fourth State of Matter", Ed. by F. D. Saeva (Marcel Decker, 1979); G. W. Gray, "Molecular Structure and the Properties of Liquid Crystals" (Academic Press, 1962); "Liquid Crystal Devices", Ed. T. Kallard (Optosonic Press, 1973); "Liquid Crystals and Their Applications", Ed. T. Kallard (Optosonic Press, 1970); and J. D. Margerum and L. J. Miller, "Electro-Optical Applications of Liquid Crystals", J. Colloid and Interface Science 58, pages 559-580 (1977).
Birefringent materials have been known and used for controlling visible light, and there are many potential applications for such materials in other portions of the electromagnetic spectrum such as the microwave range. Although viable birefringent and Kerr-effect materials are known for use in the visible-light frequency range, until recent years there have been known few if any practical Kerr-effect materials nor any birefringent materials for microwave radiation applications. The most promising reported Kerr-effect materials for use in the microwave range are suspensions of highly asymmetric metallic particles in organic liquids. It is, however, difficult to maintain a stable suspension of the high concentration of particles required for practical applications. Mechanical agitation (by circulation of the fluid) has been used in the microwave studies of these Kerr media, but continuous mechanical agitation is not practical for many device applications. The birefringence of these suspensions, when achieved, is typically about 0.08 or less.
Although such media having controllable indices of refraction to microwaves are known, there is a need for much more stable media with increased birefringence and convenience of use to control beams of microwave energy, in applications such as scanning array antennas. The greater the birefringence, the more readily a device can be made to achieve a preselected degree of beam control. Moreover, the cost of the medium and the control mechanism for many existing microwave modulators, such as ferrite phase modulators, is so high that they cannot be used in applications that require a large number of arrays, or must be low cost to be commercially feasible. The present invention fulfills the need for an improved microwave-birefringent material which can be controlled with applied electric and magnetic fields, and further provides related advantages.